Rmail reformats the header of each message before displaying it for
the first time. Reformatting hides uninteresting header fields to
reduce clutter. You can use the t command to show the entire
header or to repeat the header reformatting operation.

t

Toggle display of complete header (rmail-toggle-header).

Reformatting the header involves deleting most header fields, on the
grounds that they are not interesting. The variable
rmail-ignored-headers holds a regular expression that specifies
which header fields to hide in this way--if it matches the beginning of
a header field, that whole field is hidden.

Rmail saves the complete original header before reformatting; to see
it, use the t command (rmail-toggle-header). This
discards the reformatted headers of the current message and displays it
with the original header. Repeating t reformats the message
again. Selecting the message again also reformats.

One consequence of this is that if you edit the reformatted header
(using e; Section 29.15), subsequent use of t will
discard your edits. On the other hand, if you use e after
t, to edit the original (unreformatted) header, those changes are
permanent.

When the t command has a prefix argument, a positive argument
means to show the reformatted header, and a zero or negative argument
means to show the full header.

When the terminal supports multiple fonts or colors, Rmail
highlights certain header fields that are especially interesting--by
default, the From and Subject fields. The variable
rmail-highlighted-headers holds a regular expression that
specifies the header fields to highlight; if it matches the beginning
of a header field, that whole field is highlighted.

If you specify unusual colors for your text foreground and background,
the colors used for highlighting may not go well with them. If so,
specify different colors for the highlight face. That is worth
doing because the highlight face is used for other kinds of
highlighting as well. Section 13.1, for how to do this.

To turn off highlighting entirely in Rmail, set
rmail-highlighted-headers to nil.

You can highlight and activate URLs in incoming messages by adding
the function goto-address to the hook
rmail-show-message-hook. Then you can browse these URLs by
clicking on them with Mouse-2 or by moving to one and typing
C-c RET. Section 31.28.2.